Over the last several years, conservation of power in computer systems, particularly personal computers, has become an important issue. Initially, power consumption was a major factor in portable computers where the overall power consumption of the computer directly affects the duration of its operation under battery power. By efficiently controlling power consumption, particularly during periods of inactivity, the battery life of a portable computer can be greatly extended.
To conserve power during periods of inactivity, portable computers take steps to reduce power to various components. Power conservation steps vary between vendors; in general, power or clock cycles are disabled in various subsystems, including the video subsystem, and the speed of the microprocessor and the spin rate of the hard disk may be reduced. For the power management techniques to be transparent to the user, the computer must be able to recover from a power management shut-down in precisely the state which would exist in the absence of power management techniques. The video subsystem contains a frame buffer whose integrity cannot be ensured once refresh cycles are disabled, which will occur if all clocks are disabled on the video subsystem or if the power to the video subsystem is disabled. To prevent frame buffer data losses due to clock disablement, some portable computers couple an additional clock, generated outside of the video subsystem to the frame buffer to perform the frame buffer refresh. This solution has several problems, particularly with desktop systems. First, system design is complicated because of the additional circuitry for enabling the clock upon transition to a power conservation mode. Second, it requires hardware dependence between the video subsystem and the system motherboard.
Prior to a power shut-down to the video subsystem, the contents of the frame buffer, typically one to two megabytes, must be stored to disk (or other non-volatile memory) prior to initiation of the power management techniques. Additionally, state information for the video controller must be saved to disk prior to disabling the video subsystem. Consequently, upon resuming normal operation in response to some activity (such as a keystroke), the stored data must be transferred from the non-volatile memory to the video subsystem to restore the state information. The time delay incurred as a consequence of the power management techniques regarding the video subsystem can be significant and, hence, frustrating to the user. Further, saving the frame buffer and state information complicates the design of the computer system.
More recently, the issue of power reduction has become more important with desktop systems, as well as portable computers. While desktop systems do not use battery power and, hence, power management is not needed for enhanced operation of the system, the greatly increased use of desktop computers has resulted in an overall increase in power consumption. Recognizing the need for energy conservation, the federal government has started the "Energy Star" program to recognize computers which meet certain energy consumption guidelines. Particularly with desktop units, it is important to perform the energy management measures without affecting the performance of the system since, aside from environmental issues, the energy management systems do not have an offsetting benefit as they do in a portable computer.
Further, since video subsystems are often coupled to the system motherboard as a card on the system bus, it is desirable that the power management features of the video card are not dependent upon system board hardware, such as auxiliary clock circuits.
Therefore, a need has arisen to provide energy management in a video subsystem which can be easily implemented and does not significantly affect the performance of the computer, while still producing significant energy conservation.